The use of phase change materials to store heat energy has been known for some time.
The potential of the salt hydrates for storage materials was recognized by M. Telkes, "Solar Energy Storage", ASHRAE Journal, September, 1974, pgs. 38-44. She identified several inexpensive salt hydrates with useful freeze-thaw characteristics. Typical salts are Na.sub.2 SO.sub.4.10H.sub.2 O, Na.sub.2 HPO.sub.4.12H.sub.2 O and MgSO.sub.4.7H.sub.2 O. The large latent heat of the ice-water transition (80 cal/g or 143 Btu/lb) suggests that salt hydrates with many water molecules will have a similarly large heat content. Although this is true in general, one problem with their use in thermal storage applications is that hydrates with large water content tend to have low melting temperatures, making it difficult to extract heat rapidly. Also, many of the hydrates melt incongruently, i.e., to a liquid and a second crystalline phase of different composition. This incongruent melting leads to segregation effects which degrade the storage capacity. Segregation effects may be avoided in the congruently melting compounds, such as hydrates of ferric chloride and sodium thiosulfate (photographer's "hypo"); compounds, which in effect, dissolve in their own waters of crystallization. Unfortunately, however, there are stability problems (hydrolysis) with these two compounds and with many other congruently melting hydrates which make them unsuitable for long-term storage.
The cerrobend eutectic is an alloy mixture with good thermal conductivity, about 10 times that of water, but materials cost makes this candidate a very unlikely choice.
The salt eutectics mentioned above are typical of a number of different compositions, most of which are in a temperature range that is too high for simple home heating. The recombination of compounds like Mg(OH).sub.2 yields reasonable quantities of heat but again requires a high temperature source to drive the reaction. The reaction must be carefully controlled to avoid a dangerous pressure built up during the conversion of the hydroxide to MgO+H.sub.2 O and is therefore not suitable for domestic use.